A preferred method of lithographic platemaking uses the so-called directly exposable plates that work according to the silver salt diffusion transfer reversal (DTR) process as such plates are positive-working systems having a high photographic speed compared to the conventional so-called pre-sensitized plates which require intense exposure to ultraviolet light through an intermediate phototypesetting film as exposure mask. Therefore, DTR lithographic printing plates show a high versatility of being imaged by projection cameras directly from the paste-up copy as well as by most commonly available laser image-setters currently used in graphic arts to expose said intermediate phototypesetting films.
The principles of the DTR process have been described e.g. in U.S. Pat. No. 2,352,014 and in the book "Photographic Silver Halide Diffusion Processes" by Andre Rott and Edith Weyde--The Focal Press--London and New York, (1972). By DTR processing of an information-wise exposed silver halide material, non-developed silver halide present in the photosensitive emulsion layer of the material is transformed with a so-called silver halide solvent into soluble silver complex compounds which are allowed to diffuse into an image receiving layer and are reduced therein with a developing agent, generally in the presence of physical development nuclei, to form a positive silver image having reversed image density values ("DTR image") with respect to the negative black silver image obtained in the exposed areas of the photographic material. In a preferred embodiment, said image receiving layer is present in the same material as the photosensitive layer, thereby forming a so-called mono-sheet DTR material, though two-sheets embodiments are also well known in the art.
A typical lithographic printing plate precursor which is suitable for DTR processing is characterized by a hydrophilic image receiving layer, being water-receptive and non-wettable by ink. The positive silver image, formed after DTR processing at the exposed areas in the image receiving layer of said printing plate, is hydrophobic and its ink-receptivity is further enhanced by hydrophobizing agents which are present in the processing solution(s). Thus after DTR processing, the exposed areas of the obtained lithographic printing plate are hydrophobic, ink-receptive printing areas and the unexposed areas are hydrophylic, water-receptive non-printing areas.
Besides a high printing endurance and stringent photographic specifications such as high contrast and resolving power, suitable DTR lithographic printing plates have to meet rigid requirements with regard to the so-called ink/water balance of the plate, i.e. a good ink-acceptance at the printing areas as well as no ink-acceptance at the non-printing areas. Especially ink-acceptance by non-printing areas, a deficiency called `staining`, is unacceptable as it results in incorrect prints. When staining occurs during start-up of the printing process, it is often called `toning` and the term `scumming` may be used for the occurrence of staining during the printing process. Toning results in a high number of copies that have to be disposed of before an acceptable print is obtained, which is a major disadvantage especially for low printing runs. Also a poor ink-acceptance at the printing areas may result in a high number of useless copies during start-up. Printing plates characterized by such a poor `inking-up` quality need a high number of unacceptable copies before a steady, perfect black print is obtained.
There is a particular need for DTR lithographic printing plates characterized by a higher hydrophilicity in the non-printing areas without deteriorating the ink-acceptance of these plates in the printing areas in order to address the following problems. While starting up the lithographic printing process, the printing plates often have to be moisturized manually which is difficult to achieve for large format four colour presses due to lack of time or geometric restrictions. Furthermore, DTR lithographic printing plates are less hydrophilic than the conventional pre-sensitized lithographic plates and therefore require dedicated dampening liquids. It would be advantageous to enable users to apply the same dampening liquids to both types of plates. Another demand for DTR lithographic printing plates is compatibility with conventional processing equipment that is coupled in line with image-setters used to expose phototypesetting film. Such processors generally comprise three `wet` sections, i.e. a developing, a fixing and a rinsing section through which the material passes before entering the dryer. When such processing equipment is used for directly exposable DTR lithographic printing plates, the developing and fixing section are filled with respectively an alkaline processing liquid and a neutralizing liquid. Rinsing is a superfluous step in the processing cycle of DTR lithographic printing plates. The additional rinsing section in said conventional processing equipment prevents that the neutralizing liquid, which normally remains on the plate's surface, dries up into the layers of the plate and as a result, the total hydrophobicity of the plate's surface increases when compared with a dedicated DTR processing equipment which does not comprise a rinsing section.
For the reasons mentioned above, it would be advantageous to provide DTR lithographic printing plates having a surface characterized by a higher hydrophilicity in the non-printing areas provided that the ink-acceptance in the printing areas is not affected adversely. It is known in the art that the hydrophilicity of a DTR printing plate's surface is highly determined by the nature of the polymer binders in the image receiving layer. Useful examples of suitable polymer binders are hydrophilic polymer derivatives of acrylic and methacrylic acid. The hydrophilicity of such (meth)acrylic polymers can range from low (e.g. polyacrylonitrile) over medium (e.g. crosslinked polyacrylic acid) to very high (e.g. water-soluble polyacrylate salts or polyacrylamide). U.S. Pat. No. 5,236,802 and EP-A-769,722 disclose that the presence of polyacrylamide in the image receiving layer of DTR lithographic printing plates enhances the hydrophilicity of the plate's surface and accordingly reduces the occurrence of staining.
The presence of polymer binders characterized by a high hydrophilicity such as polyacrylonitrile however render the printing areas less hydrophobic and reduce the inking-up quality of the plate. Therefore, the prior art materials still need to be improved with regard to the ink/water balance.